Synthesis and preclinical evaluation of a novel fluorine-18 labeled small-molecule PET radiotracer for imaging of CXCR3 receptor in mouse models of atherosclerosis

BACKGROUND

CXCR3 is a chemokine receptor and is expressed in innate and adaptive immune cells. It promotes the recruitment of T-lymphocytes and other immune cells to the inflammatory site in response to the binding of cognate chemokines. Upregulation of CXCR3 and its chemokines has been found during atherosclerotic lesion formation. Therefore, detection of CXCR3 by positron emission tomography (PET) radiotracer can be a useful tool for detecting the development of atherosclerosis in a noninvasive manner. Herein, we report the synthesis, radiosynthesis, and characterization of a novel fluorine-18 (F-18, 18F) labeled small-molecule radiotracer for the imaging of the CXCR3 receptor in mouse models of atherosclerosis.

RESULTS

The reference standard 1 and its precursor 9 were synthesized over 5 steps from starting materials in good to moderate yields. The measured Ki values of CXCR3A and CXCR3B were 0.81 ± 0.02 nM and 0.31 ± 0.02 nM, respectively. [18F]1 was prepared by a two-step radiosynthesis with a decay-corrected radiochemical yield of 13 ± 2%, radiochemical purity > 99%, and specific activity of 44.4 ± 3.7 GBq/µmol at the end of synthesis (n = 6). The baseline studies showed that [18F]1 displayed high uptake in the atherosclerotic aorta and brown adipose tissue in Apolipoprotein E (ApoE) knockout (KO) mice fed with a high-fat diet over 12 weeks. The uptake of [18F]1 in these regions was reduced significantly in self-blocking studies, demonstrating CXCR3 binding specificity. Contrary to this, no significant differences in uptake of [18F]1 in the abdominal aorta of C57BL/6 control mice fed with a normal diet were observed in both baseline and blocking studies, indicating increased CXCR3 expression in atherosclerotic lesions. Immunohistochemistry studies demonstrated that [18F]1-positive regions were correlated with CXCR3 expression, but some atherosclerotic plaques with significant size were not detected by [18F]1, and their CXCR3 expressions were minimal.

CONCLUSION

[18F]1 was synthesized with good radiochemical yield and high radiochemical purity. In PET imaging studies, [18F]1 displayed CXCR3-specific uptake in the atherosclerotic aorta in ApoE KO mice. [18F]1 visualized CXCR3 expression in different regions in mice aligned with the tissue histology studies. Taken together, [18F]1 is a potential PET radiotracer for imaging CXCR3 in atherosclerosis.

Synthesis and preclinical evaluation of a novel fluorine-18 labeled small-molecule PET radiotracer for imaging of CXCR3 receptor in mouse models of atherosclerosis

Background: CXCR3 is a chemokine receptor and is expressed on innate and adaptive immune cells. It promotes the recruitment of T-lymphocytes and other immune cells to the inflammatory site in response to the binding of cognate chemokines. Upregulation of CXCR3 and its chemokines has been found during atherosclerotic lesion formation. Therefore, the detection of CXCR3 by positron emission tomography (PET) radiotracer may be a useful tool to detect atherosclerosis development noninvasively. Herein, we report the synthesis, radiosynthesis, and characterization of a novel fluorine-18 (F-18, ¹⁸ F) labeled small-molecule radiotracer for the imaging of the CXCR3 receptor in mouse models of atherosclerosis. Methods: The reference standard ( S )-2-(5-chloro-6-(4-(1-(4-chloro-2-fluorobenzyl)piperidin-4-yl)-3-ethylpiperazin-1-yl)pyridin-3-yl)-1,3,4-oxadiazole ( 1 ) and its corresponding precursor 9 were synthesized using organic syntheses. The radiotracer [ ¹⁸ F] 1 was prepared in one-pot, two-step synthesis via aromatic ¹⁸ F-substitution followed by reductive amination. Cell binding assays were conducted using 1 , [ ¹²⁵ I]CXCL10, and CXCR3A- and CXCR3B-transfected human embryonic kidney (HEK) 293 cells. Dynamic PET imaging studies over 90 min were performed on C57BL/6 and apolipoprotein E (ApoE) knockout (KO) mice that were subjected to a normal and high-fat diet for 12 weeks, respectively. Blocking studies were conducted with preadministration of the hydrochloride salt of 1 (5 mg/kg) to assess the binding specificity. Time-activity curves (TACs) for [ ¹⁸ F] 1 in both mice were used to extract standard uptake values (SUVs). Biodistribution studies were performed on C57BL/6 mice, and the distribution of CXCR3 in the abdominal aorta of ApoE KO mice was assessed by immunohistochemistry (IHC). Results: The reference standard 1 and its precursor 9 were synthesized over 5 steps from starting materials in good to moderate yields. The measured K _i values of CXCR3A and CXCR3B were 0.81 ± 0.02 nM and 0.31 ± 0.02 nM, respectively. [ ¹⁸ F] 1 was prepared with decay-corrected radiochemical yield (RCY) of 13 ± 2%, radiochemical purity (RCP) >99%, and specific activity of 44.4 ± 3.7 GBq/µmol at the end of synthesis (EOS) ( n =6). The baseline studies showed that [ ¹⁸ F] 1 displayed high uptake in the atherosclerotic aorta and brown adipose tissue (BAT) in ApoE KO mice. The uptake of [ ¹⁸ F] 1 in these regions was reduced significantly in self-blocking studies, demonstrating CXCR3 binding specificity. Contrary to this, no significant differences in uptake of [ ¹⁸ F] 1 in the abdominal aorta of C57BL/6 mice were observed in both baseline and blocking studies, indicating increased CXCR3 expression in atherosclerotic lesions. IHC studies demonstrated that [ ¹⁸ F] 1 -positive regions were correlated with CXCR3 expression, but some atherosclerotic plaques with significant size were not detected by [ ¹⁸ F] 1 , and their CXCR3 expressions were minimal. Conclusion: The novel radiotracer, [ ¹⁸ F] 1 was synthesized with good RCY and high RCP. In PET imaging studies, [ ¹⁸ F] 1 displayed CXCR3-specific uptake in the atherosclerotic aorta in ApoE KO mice. [ ¹⁸ F] 1 visualized CXCR3 expression in different regions in mice is in line with the tissue histology studies. Taken together, [ ¹⁸ F] 1 is a potential PET radiotracer for the imaging of CXCR3 in atherosclerosis.

PET Imaging Radiotracers of Chemokine Receptors

Chemokines and chemokine receptors have been recognized as critical signal components that maintain the physiological functions of various cells, particularly the immune cells. The signals of chemokines/chemokine receptors guide various leukocytes to respond to inflammatory reactions and infectious agents. Many chemokine receptors play supportive roles in the differentiation, proliferation, angiogenesis, and metastasis of diverse tumor cells. In addition, the signaling functions of a few chemokine receptors are associated with cardiac, pulmonary, and brain disorders. Over the years, numerous promising molecules ranging from small molecules to short peptides and antibodies have been developed to study the role of chemokine receptors in healthy states and diseased states. These drug-like candidates are in turn exploited as radiolabeled probes for the imaging of chemokine receptors using noninvasive in vivo imaging, such as positron emission tomography (PET). Recent advances in the development of radiotracers for various chemokine receptors, particularly of CXCR4, CCR2, and CCR5, shed new light on chemokine-related cancer and cardiovascular research and the subsequent drug development. Here, we present the recent progress in PET radiotracer development for imaging of various chemokine receptors.

Synthesis, radiosynthesis, and positron emission tomography neuroimaging using 5-[18 F]fluoro-L-amino suberate

System xc- (Sx_c -) has emerged as a new biological target for PET studies to detect oxidative and excitotoxic stress. Notably, applications have, thus far, been limited to tumour imaging although Sx_c ^- ) may play a major role in neurodegeneration. The synthesis procedures of tosylate precursor and its translation to Sx_c - PET tracer 5[18F]fluoro-L-amino suberate by manual and automated radiosyntheses are described. A brain-PET study has been conducted to evaluate the tracer uptake into brain in healthy mice.

Influx rate of 18F-fluoroaminosuberic acid reflects cystine/glutamate antiporter expression in tumour xenografts

PURPOSE

¹⁸F-fluoroaminosuberic acid (¹⁸F-FASu) is a recently developed amino acid tracer for positron emission tomography (PET) of oxidative stress that may offer improved tumour assessment over the conventional tracer ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG). Our aim was to evaluate and relate dynamic ¹⁸F-FASu and ¹⁸F-FDG uptake with pharmacokinetic modelling to transporter protein expression levels in a panel of diverse tumour xenograft lines.

METHODS

Four different tumour xenograft lines were implanted in female athymic nude mice: MAS98.12 and HBCx3 (breast), TPMX (osteosarcoma) and A549 (lung). Dynamic PET over 60 min was performed on a small animal unit. The time-activity curves (TACs) for ¹⁸F-FASu and ¹⁸F-FDG in individual tumours were used to extract early (SUV_E; 2 min p.i.) and late (SUV_L; 55 min p.i.) standardised uptake values. Pharmacokinetic two-tissue compartment models were applied to the TACs to estimate rate constants K₁-k₄ and blood volume fraction v_B. Relative levels of cystine/glutamate antiporter subunit xCT were assessed by western blotting, and expression of GLUT1 and CD31 by immunohistochemistry.

RESULTS

¹⁸F-FASu showed higher SUV_E, whilst ¹⁸F-FDG exhibited higher SUV_L. Influx rate K₁ for ¹⁸F-FASu was significantly correlated with xCT levels (p = 0.001) and was significantly higher than K₁ for ¹⁸F-FDG (p < 0.001). K₁ for ¹⁸F-FDG was significantly correlated with GLUT1 levels (p = 0.002). v_B estimated from ¹⁸F-FASu and ¹⁸F-FDG TACs was highly consistent and significantly correlated (r = 0.85, p < 0.001). Two qualitatively different ¹⁸F-FASu uptake profiles were identified: type α with low xCT expression and low K₁ (A549 and HBCx3), and type β with high xCT expression and high K₁ (MAS98.12 and TPMX).

CONCLUSION

The influx rate of ¹⁸F-FASu reflects xCT activity in tumour xenografts. Dynamic PET with pharmacokinetic modelling is needed to fully appraise ¹⁸F-FASu distribution routes.

Poly(ADP-ribose) Polymerase in Neurodegeneration: Radiosynthesis and Radioligand Binding in ARC-SWE tg Mice

We report the synthesis, radiosynthesis, and characterization of a radioligand for poly(ADP-ribose) polymerase (PARP). PARP is of central importance in cell homeostasis, neuroplasticity, and neurodegeneration in the brain. A radiolabeled PARP inhibitor was developed and used for autoradiographic quantification of PARP protein concentration in wild-type and transgenic rodent brains ex vivo in high resolution. The binding of [³H]rucaparib was found to be confined to PARP-expressing domains, for example, cerebellar cortex or hippocampal regions in both models. Saturation binding experiments confirmed selective and reversible binding to a single site ( K_d = 1.1 ± 0.2 nM).

Stereospecific radiosynthesis of 3-fluoro amino acids: access to enantiomerically pure radioligands for positron emission tomography

A variety of substituted non-racemic aziridine-2-carboxylates equivalent to amino acids were prepared and subjected to ring opening reaction by [18F/19F]fluoride. The regio and stereospecific ring opening depends on the substituents on the nitrogen as well as both the carbons of aziridines. The applicability of the methods to afford access to 3-[18F/19F]fluoro amino acids are illustrated.